The present invention relates to a process for surface pre-treatment of plastic by means of an electrical corona discharge which takes place between voltage-conducting electrodes and a grounded counter-electrode, between which the plastic is located. The invention also relates to a device for carrying out this process.
In many cases, the usually smooth surface of plastics, in particular of films, presents difficulties inasmuch as the film sheets have extremely good slip and this causes them to easily tend to telescope during winding up on a reel. Further difficulties due to the smooth surfaces of films or plastics arise during processing of these materials in order to increase the adhesion of printing inks, paints, adhesives, vapor-deposited metals and the like. In order to overcome these difficulties, the prior art includes carrying out chemical-physical modification of the surface of plastics, in particular of films. A basic process, which only causes changes on the plastic surface, comprises pretreating a plastic surface by means of an electrical corona discharge.
Thus, in accordance with German Offenlegungsschrift No. 3,247,795, corona pre-treatment of a plastic film sheet is carried out by subjecting the upper side and/or the lower side of the film sheet to the action of a corona, giving different pre-treatment intensities. For this purpose, the film sheet to be treated is passed over an electrically grounded roller, and electrical charging is carried out by subjecting the side of the film sheet facing away from the roller surface to an electrical corona discharge which is produced by applying a high-frequency, high-voltage alternating current to an electrode arranged at a distance from the roller. The pre-treatment is generally carried out under air at atmospheric pressure.
The constantly increasing market demands for products having improved surface properties have also led to the development of processes using chemically reactive substances which, for example, break certain chemical bonds in the surface and thereby modify the surface properties of plastics. In U.S. Pat. No. 3,142,630, a process is described for increasing adhesion, in which a film sheet is passed through a non-ionizing liquid and, in the liquid, is subjected to a corona discharge. This liquid can be, for example, a transformer-cooling oil, vegetable oil or another pure oil, which is free of impurities and is substantially non-electroconducting.
In British Patent No. 938,325, a process is described for the pretreatment of thermoplastic films in which an electrical corona discharge takes place on the surface in a nitrogen atmosphere. The nitrogen is passed into the corona-discharge zone via distribution lines through hollow electrode lines.
In the device which is described in U.S. Pat. No. 3,274,089, organic compounds from the group comprising polymerizable organic compounds, non-polymerizable organic compounds having substitutable hydrogen atoms, and perhalogenated hydrogen are passed into the corona-discharge zone through distribution lines in order to modify the surface of the film sheets or objects made of polymers.
Common to these known processes is that reactive gases are passed into the corona-discharge region between the electrodes, or the corona discharge is carried out in a non-conducting liquid.
Japanese Patent No. 17,747/73 discloses a device in which a film surface is subjected to a corona discharge. The electrode connected to the generator comprises porous sinter metals and several metal nets. The discharge electrode is shaped so that liquid fed to it is collected and stored. The stored liquid is converted into the gas phase by the voltage applied to the discharge electrode and leaves the porous sinter metals in the form of gas particles, which move toward the film surface under the influence of the electrical field lines of the corona discharge.
In devices and processes which include a liquid as the discharge electrode in the corona-discharge operation, the necessity arises to fall back on specific devices which make possible storage or collection of the liquid and, in addition, must comprise a material which permits passage into the corona-discharge zone of the liquid which has been converted into the gas phase. If the film sheet to be pre-treated is passed through a liquid in which corona discharge takes place, the transport speed of the film sheet through the liquid is obviously limited. If corona discharge takes place in a reactive atmosphere on the plastic surface, various layers can subsequently be applied by further process measures in order to finish the plastic surface. Simultaneous coating and pre-treatment is not possible in this type of case. The same applies to pre-treatment of plastic surfaces where corona discharge takes place on the surface in a liquid.